diff --git a/include/nlohmann/detail/input/binary_reader.hpp b/include/nlohmann/detail/input/binary_reader.hpp
index c070859c6..e938e42c5 100644
--- a/include/nlohmann/detail/input/binary_reader.hpp
+++ b/include/nlohmann/detail/input/binary_reader.hpp
@@ -2072,7 +2072,7 @@ class binary_reader
                             {
                                 return false;
                             }
-                            result = number;
+                            result = static_cast<std::size_t>(number);
                             return true;
                         }
                         case '[':
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 697f26745..80e367588 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -10448,7 +10448,7 @@ class binary_reader
                             {
                                 return false;
                             }
-                            result = number;
+                            result = static_cast<std::size_t>(number);
                             return true;
                         }
                         case '[':